This is safe but to me thats way below what this psu can deliver. There's 3 pcie slots and a max power rating of 750w/62.5A at 12v, so thats 250w from each pcie port safely which still has room for overdraw. using 2x570s with undervolt from one output is not an issue based on corsair standards. by numbers it can even go to 3

You should be more careful about the riser connection though, each riser can draw max 75w on 6pins and the per +12v cable draw limit is ard 54w (only 1 cable for sata but 3 for pcie, thus the ability to split to 2 gpus). so i would split the riser cables and plug into 3 separate peripheral and sata ports. 2 risers to 1 port would be hot, 3 would last u below 1 week before it melts at the ports.